Atomic variables

Definition

Atomic classes such as AtomicInteger update one value safely without a lock, using a hardware instruction called compare-and-swap. LongAdder is even faster when many threads only add to a counter.

Example code

Counting from many threads without locks

import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.LongAdder;

class Demo {
    public static void main(String[] args) throws InterruptedException {
        AtomicInteger counter = new AtomicInteger();
        LongAdder adder = new LongAdder();

        Runnable work = () -> {
            for (int i = 0; i < 100_000; i++) {
                counter.incrementAndGet();                   // atomic
                adder.increment();                           // atomic, scales better under heavy contention
            }
        };
        Thread a = new Thread(work), b = new Thread(work);
        a.start(); b.start();
        a.join(); b.join();

        System.out.println(counter.get() + " " + adder.sum());

        AtomicInteger stock = new AtomicInteger(5);
        System.out.println(stock.compareAndSet(5, 4));       // only changes it if it is still 5
        System.out.println(stock.compareAndSet(5, 3));       // fails: the value is now 4
    }
}

// Output:
// 200000 200000
// true
// false